Chemically Assisted Precompression of Hydrogen Molecules in Alkaline-Earth Tetrahydrides

Through a series of high pressure diamond anvil experiments, we report the synthesis of alkaline earth (Ca, Sr, Ba) tetrahydrides, and investigate their properties through Raman spectroscopy, X-ray diffraction, and density functional theory calculations. The tetrahydrides incorporate both atomic and quasi-molecular hydrogen, and we find that the frequency of the intramolecular stretching mode of the units downshifts from Ca to Sr and to Ba upon compression. The experimental results indicate that the larger the host cation, the longer the bond. Analysis of the electron localization function (ELF) demonstrates that the lengthening of the H–H bond is caused by the charge transfer from the metal to and by the steric effect of the metal host on the H–H bond. This effect is most prominent for BaH4, where the precompression of units at 50 GPa results in bond lengths comparable to that of pure H2 above 275 GPa.

the tetrahydride structures were carried out using ultrasoft pseudopotentials generated by CASTEP on-the-fly. A cutoff energy of 700 eV and a k-point grid spacing better than 0.04Å −1 gave well converged energies and forces for all the structures.
Other exchange-correlation functionals were tested: BLYP 15 and,the semi-empirical dispersion correction (SEDC) method of Tkatchenko and Scheffler 16 to account for the van der Waals interactions. Table S1 contains the structural parameters for the three alkaline-earth metal hydrides at 25 GPa obtained with the different functionals. BLYP volumes are bigger than those obtained with the PBE functional by 3-5-4.6 %, whereas the c/a radii and intramolecular distances are lower by 1-9-2.6 % and 3.2%. The inclusion of the van der Waals contributions leads to a considerable decrease of the volume by up to 8 Table S1: Volumes per alkaline-earth metal, c/a radii and intramolecular distances for the I4/mmm structures of the hydrides at 25 GPa.
Random structure searches were performed with AIRSS to explore the BaH 4 landscape. Ab initio molecular dynamics (AIMD) calculation in CASTEP were run with the NPT ensemble and with Barium assigned a fictitious mass number of 2 to more effectively sample the configuration space. A topological analysis of the electron density and the electron localization function was done with the CRITIC2 code 17 with the energy cutoff increased to 1000 eV to assure convergence.
In Table S2 we present the structural parameters for I4/mmm BaH 4 at 40 GPa estimated with PBE, BLYP and PBE+TS, and we included the experimental data. It can be seen that the choice of a different functional does not solve the discrepancy for c/a between theory and experiment, with an even lower c/a using BLYP.    Table S4: PBE calculated volume per metallic atom in the I4/mmm structure for the alkalineearth tetrahydrides versus pressure analysis in real space of the electron localization, 19 and later reinterpreted by Savin in terms of the Pauli kinetic energy density (t p ). 20 By definition, ELF is a relative measure of the electron localization with respect to the homogenous electron gas (HEG). In general, the ELF value approaches 1 in regions of the space where electron pairing occurs (e.g., atomic shells, bonds and lone pairs).
In analogy with QTAIM, a partition of the space based on the ELF can be performed. It consists of non-overlapping basins with well-defined chemical interpretation (cores, bonds, lone pairs).
Moreover, the basin charges come from integration of the electron density within these regions.
We have also calculated the radius of the H 2 ELF basin along the line joining its middle point and the alkaline-earth metal. It corresponds to the distance from the middle point to the minimum of the ELF profile along the line connecting it to the alkaline-earth metal.
Regardless of the Ca/Sr/Ba host and structure, the positions of the hydrogens are dictated by the electronic structure of the host lattices, as we find that hydrogens in the tetrahydrides are located on the ELF maxima of the empty metallic lattices (see Fig. 2).
Because I4/mmm is a subgroup of f cc and bcc, f cc     Figure S2: Comparison between experimentally derived and calculated lattice parameters for the tetrahydrides. a) Volume per Ba/Sr/Ca atom as a function of pressure. b) Lattice parameters for the I4/mmm structure as a function of pressure. c) c/a ratios for the I4/mmm structure as a function of pressure.   Figure S4: X-ray diffraction patterns of Calcium and hydrogen compressed to 52 GPa, before (bottom) and after laser heating (top). CaH 2 is refined to hexagonal P6 3 /mmc and CaH 4 is refined to tetragonal I4/mmm (λ = 0.2904Å). Tick marks indicate Bragg peaks from the labelled phases.   Figure S5: a) Representative X-ray diffraction patterns of strontium hydride before laser heating (43 GPa), after laser heating (44 GPa) and upon decompression. SrH 4 is refined to tetragonal I4/mmm (λ = 0.2922Å). Tick marks indicate Bragg peaks from the labelled phases. b) Representative X-ray diffraction patterns of barium hydrides. BaH 4 is refined to I4/mmm upon compression to 45 GPa and on decompression to 35 GPa. At 17 GPa BaH 4 is refined to the f cc phase (λ = 0.2922Å). c) Representative X-ray diffraction patterns of barium hydrides upon decompression from the Ba 8 H 46 phase. 25 At 18 GPa and below, the patterns are refined to f cc BaH 4 (λ = 0.4115Å). Asterisks correspond to BaH 2 .  Figure S6: Rietveld refinement of the XRD patterns collected from an Sr + NH 3 BH 3 experiment, collected before (top) and after (middle and bottom) laser heating at a pressure of 74 GPa. After the first laser heating SrH 2 is formed and after a subsequent laser heating the patterns can be indexed to an fcc structure of which the lattice parameters and volumes agree with the previously reported The larger distortion seen for I4/mmm BaH 4 (c/a > 2) leads to a more elongated octahedron consistent with the larger H-H distance. b) R3m BaH 4 , a theoretical stable structure found in AIRSS with the molecule oriented on ( 1 2 , 1 2 , 1 2 ); this has a more regular octahedron, similar to the experimentally observed Fm3m with a higher ELFvalue at the octahedral maximum than I4/mmm. Other stable structures found by AIRSS have similar Ba sublattices with various other molecular orientations and associated distortions of the octahedral site. With enforced I4/mmm symmetry a local minimum energy structure exists with c/a = 2.3, no molecular hydrogens, and each Ba in a cage of 18 hydrogens. This BaH 4 I4/mmm structure becomes more stable than the molecule-containing version at higher pressures around 160 GPa (Ba 8 H 46 is still the ground state). Figure S11: Schematic representation of the librational mode (left) and the H-H vibrational stretching mode (right) within I4/mmm tetrahydrides.